The present disclosure relates to composite aircraft structures, such as fuselages, in general, and in particular, to composite aircraft structures incorporating closed hat stiffeners that enable the thickness of the skins of the structures to be minimized while providing acceptable levels of impact resistance and damage tolerance, thereby reducing the weight and cost of such structures.
Composite aircraft structures offer many substantial advantages for the commercial aircraft industry. Several design considerations are critical to a successful and safely designed composite aircraft structure. Two of those design considerations, impact resistance and damage tolerance, are critical driving factors for weight and cost. Because of these two design requirements, composite fuselage skin structures may utilize a minimum gage that is much thicker than needed for carrying vehicle loads.
In general, closed “hat” stiffeners provide great torsion rigidity, bending stiffness, and buckling resistance in composite structures for airplane applications, such as fuselages. Closed hat-stiffened composite structures typically offer lighter structural weight with less material and manufacturing cost compared with “open” hat sections or J section stiffeners such as are typically used for frames. However, prior art hat-stiffened structure concepts only allow for hat stiffeners extending in the fore-aft direction, and stiffeners extending in the other direction must be designed, fabricated, and assembled altogether differently.
For example, the frames and shear ties on one composite bodied aircraft must be cured in two separate processes and then fastened together with a large number of fasteners. The frames are currently made using a form of “resin transfer molding,” while the shear ties are a tape laminate, which entails cutting to shape, drape forming, and autoclaving to cure. As a result, extra weight, cost, manufacturing processes, and assembly time are added to the structure and overall production cost. With the current design, the so-called “fail-safe chord” of the frame is suspended slightly above the skin by means of the shear ties. The shear ties have cutouts at stringers to provide for stringer continuity. In case of a skin crack, the fail-safe chord provides an alternative load path for the hoop loads in the areas where the frame passes over the stringer, but is not as effective as it would be if tied directly to the skin. In the current design, the frame load path is thus not optimal to insure the stringer continuity.
What is needed, then, is a composite panel structure design in which the stringers and frames do not intersect each other on the same side of the structural skin, thereby creating an optimal load path for both frame and stringers. This also eliminates the need for shear ties and stringer holes at the stringer-frame intersections and minimizes the number fasteners needed at each stringer-frame intersection for fail-safety.